Method and apparatus for controlling a brake system of a vehicle

Abstract

As part of an automatic drive slip control system, the brake pressure in the wheel brake of a wheel which is starting to slip is built up and released. A valve which brings about the release of pressure in this wheel brake is actuated at timed intervals so as to maintain a desired pressure level during a control phase.

I claim: 
     
       1. Method for controlling a brake system of a vehicle having wheels including a drive wheel, a wheel brake associated with one of said wheels, an inlet valve and an outlet valve assigned to said wheel brake, and a pressure pump increasing pressure in said wheel brake, said method comprising: determining slip of the drive wheel;   actuating said pump and opening said inlet valve to supply pressure to the wheel brake responsive to a determination that said drive wheel is slipping;   responsive to a determination that the slip of the drive wheel is within a predetermined range, opening said outlet valve for predetermined time intervals while said pump remains actuated and said inlet valve remains open so that the pressure in said wheel brake is held substantially constant;   reducing the pressure in the wheel brake by closing the inlet valve and opening the outlet valve responsive to a determination that the slip of the drive wheel is below a predetermined limit value.   
     
     
       2. Method according to claim 1, wherein the intermittent opening of the outlet valve occurs during the course of an automatic control phase in which the pressure in the wheel brake is kept at a constant average value. 
     
     
       3. Method according to claim 1, wherein the wheel brake has a wheel brake cylinder and a connection with a master brake cylinder having a reservoir, and, during the intermittent opening of the outlet valve, the connection between the wheel brake cylinder and the master brake cylinder or its reservoir is cut off. 
     
     
       4. Method according to claim 1, wherein the pressure in the wheel brake is modulated by the intermittent opening of the outlet valve, pressure medium being sent to a storage chamber when the pressure is being released, and volume being taken from the storage chamber and sent by the pump to the wheel brake cylinder when pressure is being increased. 
     
     
       5. Method according to claim 1, wherein the wheel brake has a wheel brake cylinder with a connection with a master brake cylinder, and, when the slip exceeds a predetermined limit, the connection between the wheel brake cylinder and the master brake cylinder is opened to build up the pressure. 
     
     
       6. Method according to claim 1, wherein the wheel brake has a wheel brake cylinder with a connection with a master brake cylinder, and, to release the pressure, a switching valve in said connection between the wheel brake cylinder and the master brake cylinder is opened briefly. 
     
     
       7. Method according to claim 1 wherein two of the wheels are drive wheels to which automatic slip control is applied, said drive wheels each being associated with wheel brakes with inlet and outlet valves associated therewith for applying pressure thereto, and said method further comprising determining when adhesion coefficients at the two drive wheels to which automatic slip control is being applied are different,   detecting which of the brakes is subjected to greater pressure and which of said brakes is subjected to lower pressure; and   responsive to determination of said different adhesion coefficients, intermittently opening the outlet valve at the wheel brake subjected to the greater pressure, whereas, at the wheel brake with the lower pressure, the pressure level is maintained by a different control strategy.   
     
     
       8. Method for controlling the brake system of a vehicle having wheels, two of the wheels being drive wheels to which automatic slip control is applied, said drive wheels each being associated with wheel brakes each having a wheel brake cylinder and inlet and outlet valves associated therewith for applying brake pressure thereto, said method comprising: when one of the drive wheels is experiencing an elevated level of slip, braking said drive wheel,   the brake pressure being built up and released in the wheel brake cylinder of the drive wheel with increased slip so that the slip of said drive wheel is reduced,   a valve bringing about the release in pressure in the wheel brake cylinder in question being actuated at timed intervals so that the pressure is first released and then built back up again to maintain the slip of the wheel within a certain range,   determining when adhesion coefficients at the two drive wheels to which automatic slip control is being applied are different,   detecting which of the brakes is subjected to greater pressure and which of said brakes is subjected to lower pressure; and,   responsive to determination of said different adhesion coefficients, the outlet valve at the wheel brake subjected to the greater pressure is opened intermittently, whereas, at the wheel brake with the lower pressure, the pressure level is maintained by a different control strategy, and   altering the intermittent opening of the outlet valve to the different control strategy based on a pressure difference between the wheel brakes of the two drive wheels.   
     
     
       9. Apparatus for controlling a brake system of a vehicle, having wheels including a drive wheel, a wheel brake associated with one of said wheels, an inlet valve and an outlet valve assigned to said wheel brake, and a pressure pump increasing pressure in said wheel brake, said apparatus comprising: means for determining slip of the drive wheel;   means for actuating said pump and opening said inlet valve to supply pressure to the wheel brake responsive to a determination that said drive wheel is slipping;   means for opening, responsive to a determination that the slip of the drive wheel is within a predetermined range, said outlet valve for predetermined time intervals with said pump actuated and said inlet valve open so that the pressure in said wheel brake is held substantially constant;   means for reducing the pressure in the wheel brake by closing the inlet valve and opening the outlet valve responsive to a determination that the slip of the drive wheel is below a predetermined limit value.